Application of a nitride semiconductor to a semiconductor device with high-voltage resistance and high output, by using the features of the nitride semiconductor, which has a high saturation electron speed, a wide band gap and the like, is being investigated. For instance, a band gap of GaN, which is a nitride semiconductor, is 3.4 eV, which is larger than a band gap (1.1 eV) of Si and a band gap (1.4 eV) of GaAs, and accordingly GaN has high breakdown field strength. For this reason, GaN is very promising as a material for a semiconductor device for a power supply for obtaining high-voltage operation and high output.
As for the semiconductor device using the nitride semiconductor, there have been a large number of reports on field effect transistors, and particularly high electron mobility transistors (High Electron Mobility Transistor: HEMT). For instance, in a GaN-based HEMT (GaN-HEMT), an AlGaN/GaN-HEMT which uses GaN as an electron transit layer and AlGaN as an electron supply layer has received attention. In the AlGaN/GaN-HEMT, distortion occurs in AlGaN, which originates in the difference between the lattice constants of GaN and AlGaN. A high concentration of two-dimensional electron gas (2D EG) is obtained due to the piezo polarization generated by the distortion and the spontaneous polarization of AlGaN. For this reason, the nitride semiconductor is expected to be a highly efficient switching element and a high-voltage resistant power device for electric vehicles and the like.    Patent Literature 1: Japanese Laid-open Patent Publication No. 2010-153493    Patent Literature 2: Japanese Laid-open Patent Publication No. 2009-49288    Patent Literature 3: Japanese Laid-open Patent Publication No. 2008-71988
However, the GaN-HEMT generally has defects of having no avalanche resistance and being considerably weak at a surge. In addition, the GaN-HEMT does not have a body diode therein, which is different from a Si-based semiconductor device, and needs to have a diode connected to the outside as a so-called free wheel diode (FWD), in order to be applied to an inverter circuit (full-bridge inverter circuit) or the like, for instance. In addition, the GaN-HEMT has a large ratio Cgd/Cgs of a parasitic capacity Cgd between a gate electrode and a drain electrode to a parasitic capacity Cgs between the gate electrode and a source electrode, in comparison with the Si-based semiconductor device, and there is a concern that the operation tends to be unstable.